Various devices have heretofore been used to protect loudspeakers connected to the outputs of high power audio amplifiers from excess signal amplitudes. One expedient used for this purpose is a limiting circuit which clips the output of the amplifier at a level which is not dangerous to the loudspeaker. Another expedient which is sometimes employed is a compressor circuit. When a limiter or clipping circuit is employed, distortion results since waveforms are always clipped at some predetermined level. Compressor circuits, on the other hand, are found to have undesirable listening characteristics.
In accordance with the present invention, the assumption is made that the speakers used with high power amplifiers can accommodate frequent peaks well above their nominal power ratings, provided these occur for short intervals only. It is, therefore, desirable to provide a protective circuit for loudspeakers which permits peaks to pass through, thereby retaining fidelity of reproduction. It is, however, also important to reduce the inputs of the loudspeaker when sustained high level outputs occur which are beyond the longer term capability of the loudspeaker. It is also desirable to provide a visual indicator of the fact that the loudspeaker is being subjected to nearly overload conditions, in order that ameliorative action may be taken before actual overload occurs.
In accordance with the present invention, no distortion frequency response or output level changes are introduced in the output of a power amplifier, so long as the long term voltage applied to the loudspeaker fails to exceed a predetermined set level, and despite the fact that short peaks of excess voltage may occur. Upon sensing a sustained overload, by means of an integrating circuit for a peak voltage output sensing device, controlled shunting effect is introduced at the input of the amplifier, so that the net signal applied to the input of the amplifier is reduced only enough to avoid sustained overvoltage. No distortion occurs on short term overvoltage, but rather the peak input to the amplifier is reduced in response to long term overvoltage to a level which provides an output that the loudspeaker can sustain. When the integrated output voltage attains a predetermined value, input signal by-passing action commences, but the extent of by-passing is a function of the extent of overvoltage. Just before the selected level is achieved, an overload indicating lamp is illuminated. By means of the present system, the amplifiers can pass to the loudspeaker very short peaks up to the level at which the amplifier clips, and it can pass longer duration peaks of intermediate power, so long as the average power level is substantially below the set threshold. As power output rises the extent of by-pass is increased, but only sufficiently that the loudspeaker is fully protected. The overload lamp can be turned on about 1 db below the threshold at which protective signal limiting commences. The circuit does not latch in, and shortly after cessation of the overdrive, full input signal is restored to the main amplifier, by-passing being continued only until the integrated signal level drops below the threshold. At that point the indicating lamp has become extinguished.